Enslaved devices are known to adapt their actions, for example, the stimulation frequency in the case of a cardiac pacemaker, to the measured value or a value calculated value from a representative parameter of metabolic needs of the wearer of the device. In this regard, the term "enslaved" should be generally understood to mean a device having a mode of operation that senses a parameter and operates according to a function that relates the sensed parameter to a desired operating condition. Most typically, one refers to a pacemaker that is enslaved to a physiological parameter, meaning that it has a sensor that senses a physiological parameter indicative of the patient's cardiac output requirements, and then implements a pacing rate that is determined as a function of that parameter. Such pacemakers are also referred to as rate responsive or rate modulated pacemakers, because the pacing rate varies (or is modulated) according to the sensed needs of the patient.
EP-A-0 089 014 describes the utilization of the measure of the respiratory frequency (breathing rate) to vary the instantaneous cardiac stimulation frequency. Several other parameters, such as the minute ventilation (also known as minute volume), the saturation of oxygen in the blood, the blood or body temperature and the acceleration (e.g., physical motion) have been used as enslavement (rate modulation or rate responsive) parameters.
In the case of cardiac pacemakers, all these systems operate to increase the frequency of stimulation pulses when one detects an increasing activity of the patient wearing the device (i.e., the patient in which the device is implanted or on which the device is carried), and to decrease this frequency to a base value in the case of a diminution of activity, particularly during phases of rest of the patient.
EP-A-0 493 222 describes a process of correlation between, on the one hand, the two extreme values Fc.sub.base and Fc.sub.max of the range of the stimulation frequency and, on the other hand, value X.sub.base and X.sub.max, which are respectively the rest value and the value of maximal activity, calculated from information collected by the enslavement sensor. This process of correlation is known under the name of "automatic calibration of the enslavement", and the document describes a process to determine the value of X.sub.base in the case of the utilization of the minute--ventilation as the parameter of enslavement. The value of the minute--ventilation at rest is then called "VE.sub.REPOS ". This last value is obtained by the calculation of an average value during an interval on the order of 24 hours, including, therefore, periods of activity as well as periods of sleep of the wearer of the device.
The inventors have nevertheless observed and recognized that, during phases of sleep, the values of VE.sub.REPOS can be more than 50% below the values of this same parameter recorded during periods when the patient is awake (i.e., conscious) and active.
In the aforementioned document, such important variations are not and cannot be taken into consideration. Nevertheless, this value of VE.sub.REPOS is used for the automatic calibration of the enslavement of the device during the adjustment of the operating point relative to the minimal stimulation frequency FC.sub.base.
THUS, as the inventors have realized, a false or incorrect estimation of the value VE.sub.REPOS can result, therefore, in an adjustment of the stimulation frequency that is not properly related to real needs of the wearer of the device.